how do I add missing section to a cylinder
Duplicate (ShiftD) a section of the label large enough to fill the gap, (don't move it,) and then rotate it around Z until it does. 

Do the rotation in vertex select mode: select all the vertices in the new section, finishing with one of the vertices that's going to lie on the seam, to set it active. Set Snap On to 'Vertex' and 'Active'. The active vertex can the snap to its partner on the other side during a constrained rotation (RZ)

If the label is not conveniently centered on the world origin , choose 2 diametrically opposite vertices from it, and (ShiftS) send the 3D Cursor to Selection. Then use that as your rotation pivot.

Once the vertices on the seams are coincident, CtrlV bring up the vertex menu, and select 'Remove Doubles'.